1. Field of the Invention
This invention relates to a method for suppressing the production of undesirable by-products in hydrocarbon cracking processes. More particularly, this invention relates to a method for suppressing the production of light gas by-products ordinarily produced by thermal-initiated cracking reactions. In accordance with the method of the present invention, low concentrations of highly siliceous materials, having high surface area and low acidity are dispersed within the hydrocarbon feed prior to cracking.
In the petroleum industry, the partial decomposition of hydrocarbons to those of lower molecular weight is of great importance. It was discovered early on that higher boiling hydrocarbons could be broken down or cracked, into lower boiling hydrocarbons by subjecting the former to high temperatures for extended periods. During the nineteenth century, a form of cracking was used to convert heavier crude oil fractions into kerosene. Use of cracking to produce gasoline began in 1913 and greatly increased as the automobile became popular.
Cracking which is effected by heat alone is known as thermal cracking. Thermal cracking requires temperatures ranging from about 400.degree. to 650.degree. C. (750.degree. to 1200.degree. F.). High pressures (350-1000 psi) are generally required to keep the feedstock in the liquid phase while it undergoes cracking. High pressure can also prevent vaporized hydrocarbons from being over-decomposed, forming light gases and laying down coke-like solid deposits inside the cracking unit. However, high temperatures and pressures have been found unnecessary when cracking is conducted in the presence of a catalyst. Suitable cracking catalysts include naturally occurring clays or synthetic compounds which contain silica and/or alumina. Such catalysts must provide a large surface area on which the cracking reactions can occur.
Delayed coking is another important refining process involving the breakdown of higher-boiling hydrocarbons to those of lower molecular weight. Coking is basically a thermal conversion process in which the low hydrogen-to-carbon ratio components of the residuum are converted to coke. In this process, heavy residual material residues are upgraded into more valuable distillate products and coke. A wide variety of chargestocks can be utilized in this process including full range or reduced crude oils, coal tar pitch, thermal tars, and asphalt, as well as aromatic and refractory stocks, such as catalytic cycle oils. Among the resulting products are coke, heavy and light gas oils, butane-butylene, propane-propylene, and various combinations of C.sub.3 to C.sub.5 fuel gas hydrocarbons.
In the delayed coking process, the charge material is rapidly heated to a temperature greater than about 482.degree. C. (900.degree. F.). The heated feed is then conducted to one or more coking drums for an extended period during which the breakdown into coke and other products occurs. Since the process is endothermic, sufficient heat is supplied to maintain the contents of the drums between about 438.degree. C. and 466.degree. C. (820.degree. to 870.degree. F.). When the coke reaches a predetermined level in the drum, the drum is decoked with high pressure water jets. The coke drum overhead vapor enters a fractionating tower for separation into gas, gasoline and gas oils.
High temperature visbreaking is another process whereby residual hydrocarbons are broken down into more useful products under conditions of high temperature and pressure. Vacuum residuum is conducted to a furnace and there heated to temperatures ranging from about 454.degree. C. to 482.degree. C. (850.degree. to 900.degree. F.). The heated residuum is subsequently quenched with light gas oil and transferred to a flash zone in a fractionator tower. The flashing procedure breaks the residuum into dry gas (less than about 3% by weight), gasoline (about 3-10% by weight), gas oil (about 10-20% by weight) and visbroken residuum having a boiling point higher than about 343.degree. C. (650.degree. F.) (about 68-86% by weight). The product proportions can be varied by altering the reaction conditions.
Thermal cracking, delayed coking, and high temperature visbreaking processes all produce light gases, i.e., the C.sub.1 to C.sub.2 hydrocarbons such as methane, ethane and ethylene. In order to derive the greatest economic benefit from a feedstock, it is desirable to minimize the production of low molecular weight hydrocarbons products in order to maximize production of more valuable longer chain hydrocarbons. In view of the enormous amounts of crude oil which are treated by cracking, coking, or visbreaking processes, even a slight reduction in light gas produced would result in a significant economic advantage.
Two competing reactions are believed to occur in the above thermal processes. Carbonium ion-promoted reactions tend to produce molecules having three or more carbon atoms, while free radical reactions generally form one or two carbon molecules, such as methane and ethane. Because the more economically desirable molecules are promoted by the carbonium ion reaction, a method for suppressing the competing free radical reactions would likely reduce production of less desirable lighter hydrocarbon products. Such a reduction in free radical-promoted reactions would serve to increase the overall yield of C.sub.3 or greater molecules depending on the cracking temperatures.
It is known in the art to add certain components to a chargestock in order to affect the product output of a catalytic cracking process. For instance, U.S. Pat. No. 3,849,291 to Owen discloses a method of high temperature catalytic cracking with low coke-producing crystalline zeolite catalysts wherein crystalline aluminosilicate catalytic compositions are suspended in gasiform material comprising hydrocarbon reactant material. The suspended additive serves to reduce the coke make of this catalytic cracking process. However, no chargestock additive is known in the art which serves to suppress the production of light gas in high temperature cracking processes.